The present application relates to mitigating sag in drilling fluids.
While drilling a gas or oil well, a drilling fluid (also referred to as a drilling mud) is typically pumped down to the drill bit during drilling operations and flowed back to the surface through the annulus defined between the drill string and the walls of the borehole. A typical drilling fluid includes a weighting material, such as barite, to increase the density of the drilling fluid and thereby manage the wellbore pressure and also assist in transporting rock chips and cuttings from the drill bit to the surface.
Settling or migration of the suspended weighting materials within the drilling fluid is commonly referred to as “sag” or “barite sag,” and is a known and persistent problem in drilling operations. Turbulence in the moving fluid may tend to keep particles in suspension, but when the drilling fluid becomes static, such as while tripping the drill bit or when the circulation flow rate of the drilling fluid is relatively low or stopped, the weighting material(s) may tend to settle toward the bottom of the borehole.
When sag occurs in a borehole (also referred to as a sag event), it can cause borehole pressure problems that are typically manifested when the mud pumps are turned on after quiescent periods, or during operations such as tripping in when the fluids are periodically sheared and then circulation resumed. Problematic borehole pressure spikes may occur when the drilling fluid is pumped after a sag event. The lighter drilling fluid (depleted due to sag) nearer the surface is pumped out first, leaving the borehole filled with the heavier (solids accumulated portions), settled drilling fluid and the newly introduced drilling fluid that is being pumped down the drill string. As the borehole annulus is now filled with a heavier fluid, on average, and the heavier fluid may have a greater flow resistance than the original fluid and higher hydrostatic pressure because of a higher average density. The borehole pressures at some vertical depths can exceed the fracture gradient of the surrounding formation, resulting in lost circulation, formation damage and/or fracturing of the formation.
Sag is aggravated in deviated or angled boreholes due to a phenomenon called “boycott settling” or the “boycott effect.” Briefly, the boycott effect occurs since suspended particles tend to settle vertically downward, creating an increased-density or heavier layer along the lower side of the angled borehole and a reduced-density or lighter layer along the upper side. Such a pressure imbalance across the longitudinal cross-section of the angled borehole will tend to circulate the lighter layer upward and the heavier layer downward, significantly increasing the rate at which the heavier particles accumulate in the lower portion of the angled borehole. Accumulations of the weighting material in the lower portion of the borehole can be difficult to re-suspend and may cause drag on rotating drill strings or impede moving tools up or down through the region of accumulated weighting material.